Circular Gravity Thickeners

Circular Gravity Thickeners concentrate waste activated sludge (WAS) and primary sludge by allowing solids to settle while decanting clarified water from the surface. The process relies on gravity settling enhanced by slow mechanical raking to consolidate settled solids toward a central discharge point. These units typically achieve solids concentrations of 3-6% for WAS and 8-12% for primary sludge, reducing downstream digester volumes by 60-80%. The primary limitation is their large footprint requirement and sensitivity to filamentous bacteria, which can create floating sludge blankets that compromise thickening performance.

• Primary Sludge Thickening (5-50 MGD plants): Located between primary clarifiers and anaerobic digesters, CGTs concentrate primary sludge from 3-6% to 8-12% solids. Selected for high organic loading and reliable performance with minimal operator intervention. Upstream: primary clarifier underflow pumps. Downstream: digester feed pumps.

• WAS Thickening (10-50 MGD plants): Thickens waste activated sludge from secondary clarifiers from 0.5-1.5% to 3-5% solids before digestion. Chosen over DAF when polymer costs are a concern and modest thickening is acceptable. Often combined with primary sludge for co-thickening.

• Co-thickening Applications (15+ MGD plants): Simultaneously processes primary and WAS streams, achieving 6-10% combined solids. Selected for operational simplicity and reduced polymer consumption compared to separate thickening processes.

• Lime Sludge Thickening (water plants): Concentrates lime softening sludge from 2-4% to 8-15% solids. Preferred over mechanical systems due to abrasive nature of lime sludge and lower maintenance requirements.

Daily Operations: Operators monitor sludge blanket levels using interface probes or manual sounding, adjusting withdrawal rates to maintain 3-6 ft blanket depth. Torque monitoring alerts indicate rake overload conditions requiring immediate attention. Supernatant quality checks ensure proper settling, with turbidity readings typically <50 NTU for well-performing units.

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Maintenance: Monthly lubrication of drive mechanisms and quarterly inspection of rake arms for wear or damage. Annual bearing replacement and drive alignment checks prevent costly failures. Confined space entry procedures required for internal inspections. Basic mechanical skills sufficient for routine maintenance, with millwright support needed for major repairs. Typical service life: 15-20 years for mechanical components, 25+ years for concrete structures.

• Rotating Rake Mechanism: Central drive unit with radial arms and vertical pickets that slowly rotate (0.5-2 rpm). Constructed with stainless steel arms and HDPE pickets. Torque monitoring systems detect solids buildup. Municipal units range 20-150 ft diameter with 5-25 HP drives.

• Inlet Feed Well: Perforated cylinder at tank center distributes influent sludge uniformly. Typically 6-12 ft diameter with adjustable baffles. Prevents short-circuiting and hydraulic disturbances that reduce settling efficiency.

• Underflow Withdrawal System: Includes sludge blanket level sensors, variable speed pumps, and automated withdrawal controls. Maintains optimal blanket depth (3-6 ft typical) and prevents over-thickening. Features progressive cavity or centrifugal pumps sized for 2-4x average flow.

• Supernatant Overflow Weirs: Adjustable weirs maintain proper water level and collect clarified supernatant. Include scum baffles and skimming systems. Weir loading rates typically 100-300 gpd/ft for optimal performance.

• Polymer Feed System: Dilution and metering equipment for flocculant addition when required. Includes day tanks, mixers, and positive displacement pumps for precise dosing control.

• Solids Loading Rate: 6-20 lbs/ft²/day (typical municipal range 8-12 lbs/ft²/day)

• Primary sludge: 15-20 lbs/ft²/day
• WAS: 6-10 lbs/ft²/day
• Combined primary/WAS: 8-12 lbs/ft²/day

• Hydraulic Loading Rate: 200-800 gpd/ft² (typical 400-600 gpd/ft²)

• Tank Diameter: 20-150 feet (most municipal: 40-100 feet)

• 0.5-5 MGD plants: 40-60 feet
• 5-25 MGD plants: 60-100 feet
• 25-50 MGD plants: 80-150 feet

• Sidewater Depth: 8-12 feet (standard municipal range)

• Underflow Concentration:

• Primary sludge: 4-8% solids
• WAS: 2-4% solids
• Combined: 3-6% solids

• Detention Time: 1.5-4 hours (typical 2-3 hours)

• Picket Fence Speed: 1-3 rpm (variable speed drives standard)

• Torque Requirements: 10,000-50,000 ft-lbs (depends on diameter and sludge characteristics)

• Overflow Rate: <500 gpd/ft² (EPA recommended maximum)

• What is the design solids loading rate based on sludge characteristics? Primary sludge allows 15-20 lbs/ft²/day while WAS requires 6-10 lbs/ft²/day. Exceeding these rates causes poor thickening and increased polymer consumption. Need detailed sludge characterization including SVI, particle size distribution, and seasonal variations.

• Should the unit include center-feed or peripheral-feed design? Center-feed suits flows >2 MGD with better mixing but requires larger diameter. Peripheral-feed works for <5 MGD with simpler construction. Wrong choice affects thickening efficiency by 15-25%. Requires flow projections and site layout constraints.

• What picket fence configuration optimizes performance? Variable-pitch (closer at bottom) improves underflow consistency by 20-30% versus uniform spacing but costs 15% more. Deep sludge blankets (>4 feet) require closer spacing. Need pilot testing data or similar plant performance records.

• Is automated sludge blanket level control justified? Systems >10 MGD typically justify ultrasonic blanket level control ($25-40K) reducing operator attention and improving consistency. Smaller plants rely on manual control with timed withdrawal cycles.

• Primary: Division 46 - Water and Wastewater Equipment

• Section 46 21 00 - Sludge Pumping and Treatment Equipment

• Secondary: Division 40 - Process Integration (for complete systems with feed pumps, polymer systems, and controls)

• Material/Equipment Verification:

• Verify 316SS wetted parts for corrosion resistance
• Confirm torque ratings match sludge characteristics
• Review polymer feed system compatibility

• Installation Requirements:

• 18-24 month lead times typical for custom units
• Requires substantial concrete work and dewatering
• Crane access critical for mechanism installation

• Field Challenges:

• Levelness critical for proper rake operation
• Effluent weir adjustment requires precision
• Polymer system startup coordination essential

• Coordination Issues:

• Interface with existing sludge handling systems
• Electrical/control integration complexity

• Ovivo - SAGR (Superpulsator Accelerated Gravity Reactor) series, widely used in 1-50 MGD plants

• Evoqua - Gravity Belt Thickener systems, popular for smaller municipal applications

• Veolia - Densadeg clarifier-thickeners, common in larger facilities (10-100 MGD)

• WesTech Engineering - Conventional circular thickeners with center-feed designs, established municipal track record

• Dissolved Air Flotation (DAF) - Preferred for low-density sludges, 20-30% higher capital cost but better performance on waste activated sludge

• Gravity Belt Thickeners - Lower capital cost for smaller plants (<5 MGD), higher polymer usage but simpler operation

• Rotary Drum Thickeners - Compact footprint alternative, 40-50% higher operating costs but useful for retrofit applications with space constraints

Establish early relationships with manufacturer service teams - their field startup support is invaluable for optimizing polymer dosing and rake speeds. Budget 10-15% contingency for unforeseen concrete work; existing foundations rarely meet new equipment requirements. Consider standardizing on one manufacturer across multiple units to simplify spare parts inventory and operator training, typically reducing O&M costs 15-20%.